Seismic vulnerability assessment of confined masonry buildings by macro-element modeling: a case study

Confined masonry structures are seismic resisting structures, where masonry walls are confined by reinforced concrete beams and pillars. During the construction of a confined masonry structure, masonry walls are used as formworks to build the reinforced concrete elements. The reinforced concrete frame plays the important role of confining masonry walls, and therefore helps in increasing the ductility of the whole structure; this implies better performances of the confined masonry with respect to the traditional masonry structures. In confined masonry structures, openings are confined by reinforced concrete frames, while wall intersections and floor slab-wall connections are realized by means of reinforced concrete elements. As observed after several severe earthquakes, confined masonry structures showed a reliable anti-seismic behavior due to several reasons. Among them, the confinement action of the reinforced concrete frames, the in-plane floor stiffness, the plan and elevation regularity. In this paper a case study is considered, which is a building located in Reggio Calabria and built in 1932 during the reconstruction following the 1908 earthquake, which struck and destroyed a large part of the city. The original plan of this building has been found in a historical archive. Therefore information is available about the mechanical characteristics of the materials and some structural details. The seismic vulnerability of the structure is assessed according to the Italian Seismic Code by using 3DMacro structural analysis software. Masonry walls are modeled through an innovative "macro-element", which allows to take into account different collapse mechanism: bending failure (rocking), shear failure by diagonal cracking, shear failure due to sliding. Secondary element, such as pillars, beams, architraves, are modeled by using nonlinear frame element having concentrated plasticity at their ends. A critical analysis of the results has been performed, regarding both the seismic vulnerability in terms of push-over curves and the techniques to improve the anti-seismic performances.